In electrical rotating machines, electrical windings are often retained by means of wedges within slots formed in the stator or in the rotor.
The slot closing wedges may be non-magnetic or magnetic.
Magnetic wedges are obtained, for example, with dispersions of iron oxide in epoxy resin or with the sintering of ferromagnetic materials, such as iron oxides or other magnetic particles, in a binder, for example organic.
The advantages resulting from the use of magnetic wedges (that is, having a high magnetic permeability) in terms of limitation of additional leaks and dispersed flows have long been known.
The performance of an electrical motor, in particular of a permanent magnet motor, is also characterised by the extent of the harmful parameters, cogging and ripple torque.
Cogging torque is caused by the tendency of the rotor to align with the stator in the direction of maximisation of the magnetic circuit permeance whereas the ripple torque basically results from the interaction of the rotor flow with the spatial magnetic field (due to the stator winding distribution) and temporal (that is, resulting from the current waveform associated to the power converter) harmonics.
In order to reduce the cogging torque, it is known to use rotor or stator slot magnets with a slight skew relative to the direction of the rotation axis of the electrical machine. Such solution, however, may only be applied to one-piece electrical motors and not to modular electrical motors as well.
The advantages of multi-sector, that is, modular electrical machines, are known, wherein the stator is divided into separate sectors, equal to each other, and the rotor is divided into poles or polar groups separated from each other. Such electrical machines are particularly suitable for large slow windpower and submarine applications and minimise the manufacturing and maintenance costs, managing the easier-to-handle elementary components in series, and they limit any machine downtimes due to electrical failures.
It would be desirable to keep, also on modular electrical machines, the positive skew effect on the cogging and ripple torques as is possible in one-piece machines. However, as said above, the slot skewing technique is actually applicable only to one-piece machines and not also to multi-sector machines.
A further problem related to the use of magnetic wedges is represented by the leakage reactance which, especially in permanent magnet motors with wound tooth stator, is already intrinsically high and implies a low power factor. For these types of motors, the use of non-magnetic wedges is widespread, but however they cause the impossibility of having the benefits on ripple and cogging torque due to the magnetic wedge.